rx8wannahave

I was wondering...if all you wanted to use was the fan, NOT AC, would it still make the engine work harder thus waste more gas? I'm thinking it would because something has to make the fan spin...but...what are the facts with this?

Omicron

Nope, fan only just uses electrical to spin the fan. On the other hand, the AC (and the defroster, BTW), turns on the AC compressor which is driven by a belt off the engine, thus creating a parasitic drag on the engine. No compressor, no drag.

rx8wannahave

Do you mean for the front window or the rear? Front still uses the AC so I understand that but the rear uses those copper wires to do the work...so it's electrical, right?

Oh, and thanks for your reply I'm glad it does not use the AC compressor when using just the fan.

Oh, and one more question. Using the AC on level 1 compared to level 4, is there less strain in the engine? Like...just as an example:

On level 1 use get .5mpg worse fuel economy (or 1hp to run)

vs

Level 4 which gets 1.5mpg worse fuel economy (or 5hp to run)

GotZoom

Last summer we took a trip to Nashville - about 210 miles one way. We left early in the morning - had the window and sunroof open / no AC. On the way back, we had the AC on / sunroof open. I compared the mileage - if I remember correctly, had about 20 miles less at the fill up on the way back; so about 1 mpg loss.

rx8wannahave

Yeah, I know it takes away some fuel economy but I wonder how the different AC levels effect the car. What I mean is, should I turn on the AC to 3 at first to cool it down then leave it at 1 since I don't need it to cool the car as much. Would that save me some fuel economy or does it not matter if I"m in 1 or 4?

GotZoom

Can't help you there - I think we had it back and forth between 1 and 2 the whole trip. I would be curious to know what you find out.

rx8wannahave

This is my thinking…

1) To get cold air (AC) the engine has to spin the compressor but the fan speed is controlled electrically. So, the strain should be the same on the engine and it's just the fan speed that changes. With that thinking...it does not matter if you are on level 1 or 4, the engine gets it's HP and fuel economy hit either way because the compressor has to spin.

OR

2) There are different levels of speed the compressor has to spin to push out more or less cold air, so there would be different levels of strain on the engine taking more or less HP and hitting fuel economy at different levels too?

I think option 1 might be right...

Nubo

To be accurate, the fan is also demanding engine power. Just because it is electric doesn't change the fact that the power ultimately comes from the engine. (AC could run off an electric motor as well). The fan increases the electrical load on the alternator. It then takes more force to turn the alternator. Electrically powering the AC would require a substantial motor and a lot of juice, a bigger alternator, etc.

It takes FAR less energy to spin the fan than it does to spin the compressor motor. If you were to drive both from a human-powered machine (bicycle-generator setup), it would be no sweat to power the fan. Keeping the AC compressor running would be a heavy workout.

So, both cost. In practical terms though, the fan is essentially "free".

rx8wannahave

Why wont anyone answer my question???

fullsmoke

www.howstuffworks.com
Go to auto section and read about how exactly an A/C compressor works.

A/C compressors rob LOTS of power in terms of other mechanical things on a car. Through fluid mechanic properties which I'd show but might get complicated, the compressor setup in homes, cars, etc, are EXTREMELY inefficient. So just keep in mind that running your A/C robs a noticible fraction of your engine's power and reduces your MPG by quite a bit...

Now, for your question, I believe it is number 1. You'll see after reading the article why. However, I think as your increase fan speed, the engine might increase RPMs slightly in order to keep cooling the cabin at a constant temperature (slightly speed up the compressor). I can't attest to this since I've only had the 8 for a week.

Hope this helps some...
FS

robertdot

I've been running almost 100% w/o my AC since shortly after I moved to Cali. I've noticed no mileage improvements in that time.

fullsmoke

If you have studied fluid mechanics, you will understand why compressors are so inefficient. This efficiency leads to power loss to heat, etc, etc. If you want, I can probably type up proofs and explainations about efficiency of compressors. I personally saw the effects in my rx7 when I took my usual commute to Atlanta, from Huntsville (I see you are from AL). In the summer with the A/C at full blast it takes me 1.3 tanks (my 7 is not suited for long distances, hence the 8), but on a cool fall day the commute will take about .75 of a tank. I was very surprised at this finding and then remembered the days of fluid mechanics and how inefficient A/C is. I am sorry I can't attest for this in the 8 (at the moment--have only been driving it for a week), but I am speaking from an engineer's standpoint.

FS

StealthTL

iTrader: (1)

I will have a stab at Mr. Wannahave's question.......

Both 1/ and 2/ are partly correct; fan is electric, compressor is mechanical, but if you see how they interact, it may shed some light on your problem.

The compressors' set point is controlled by the temp switch, red-to-blue, but it cycles on and off controlled by the temp of the cooling coil. (so even if you set it to 'max' cool, it will only keep the coil very cold, not freeze it solid).

When you set your fan to '4' it warms the coil, causing frequent engagements of the compressors' electric clutch, and using more fuel.

If the fan is only on '1', the coil stays colder/longer, saving fuel, but probably not cooling you much.

The compressor doesn't need to go 'faster/slower' it just spins with the engine. Even at idle, when the clutch grabs, the engine slows and the ECU gives it a little more throttle opening, using more fuel, to keep the idle revs up.

Don't overlook Omicrons' point about the a/c coming on during defrost or heat cycles (it's sneaky!) That can run the compressor EVEN THOUGH the little a/c light is OFF.

One last point - the compressor swithes off at about 80% (iirc) throttle opening, to maximize WOT power when needed.

S

robertdot

RE: fullsmoke

Communism is a great theory. Not quite as great in real life. Like I said, my mileage didn't take a noticeable spike for the better once I quit using the AC. I'm sure the theory says "d00d you should have much better mileage," but the actuality says, "It doesn't matter enough to care about." At least as far as my experience...

I went to college at UAH my freshman year (commuting from Birmingham to UAH and back every weekend... then I was driving an old Mazda B Series truck...). I can think of no other reason you would drive from ATL to Huntsville (of all places) if not for school...

JM1FE

That is correct.

Driving around with windows/sunroof open creates aerodynamic drag. Using the a/c compressor with the windows closed has less aero drag, but loads the engine a tiny bit. It's not enough to stress over. Set your a/c so that you're comfortable, stop overthinking it, and drive.

rx8wannahave

LOL...OK...thanks....

fullsmoke

I live in Huntsville and commute to Georgia Tech in ATL where I currently attend school. I understand in your experience you've never seen an improvement and I'm not trying to argue that you have. Every car is different and everyone's driving habits are different. Maybe the way you drive just doesn't allow for a noticible difference. All I am trying to convey is that A/C is ineffcient and will use extra gas and robs power which is why millions of other people say: "for better gas mileage, use your A/C less."

FS